Sanaullah Ghafari, the 29-year-old leader of the Afghan branch of the Islamic State group, has overseen its transformation into one of the most fearsome branches of the global Islamist network, capable of operations far from its bases in the borderlands of Afghanistan.

The Islamic State group has claimed responsibility for Friday's mass shooting at a concert hall near Moscow that killed at least 139 people. U.S. officials have said they have intelligence indicating it was the Afghan branch, Islamic State Khorasan (ISIS-K), that was responsible.

Washington has said it had warned Russia this month of an imminent attack. A source familiar with this intelligence said it was based on interceptions of "chatter" among ISIS-K militants. Russia's foreign ministry, however, has questioned whether ISIS-K was responsible.